EFM32 (sltb001a) is missing FEATURES_PROVIDED=cpp

[Mario0x1]

Description

FEATURES_PROVIDED=cpp is not set in cpu/efm32/Makefile.features despite being able to build and run examples/riot_and_cpp

Steps to reproduce the issue

cd examples/riot_and_cpp
make BOARD=sltb001a flash

Expected results

make BOARD=sltb001a

informs that

...
There are unsatisfied feature requirements: cpp

EXPECT ERRORS!

Building application "riot_and_cpp" for "sltb001a" with MCU "efm32".
...

It should fail if RIOT-gecko-sdk does not support cpp.

Actual results

example/riot_and_cpp can be build and flashed and returns

main(): This is RIOT! (Version: 2018.04-devel-1009-g4f4e-hoss-214)

************ RIOT and C++ demo program ***********

******** Hello, you're in thread #2 ********
We'll test C++ class and methods here!
Instanciating Object [constructor called]
Im starting!

-= Test overloading functions =-
Hello!
Hello![int: 42]
Hello![float: ]

-= Test namespace =-
typing std::vector is obsolete when 'using namespace std;'
The vector vInts has been filled with 3 numbers.

-= Test iterator =-
The content of vInts = { 1 3 2 }
Destroying Object [destructor called]
Im shutting down!
******** Hello, you're in thread #3 ********
We'll test some C functions here!

-= hello function =-
Hello world in a C function

-= day_of_week function =-
day 13, month 6, year 2014 is Friday

This demo ends here, press Ctrl-C to exit (if you're on native)!

Versions

SEGGER J-Link Commander V6.22a
Gecko_sdk: https://github.com/basilfx/RIOT-gecko-sdk d381e526d68a2d0c951f37040c1c2e168ac66cd6

@basilfx

[basilfx]

I've created #9097 to address this bug.